Method for enhancing the stability of a water sensitive, reactive subterranean formation

ABSTRACT

The current invention provides a method for enhancing the formation stability of water sensitive, reactive formations penetrated by a wellbore. The method of the current invention provides an accurate evaluation of the impact of cementing fluids on water sensitive, reactive formations and provides the ability to accurately formulate cementing fluids in order to enhance the stability of such formations. When necessary, the method of the current invention additionally provides for the application of an osmotic semi-permeable membrane to the face of the formation.

BACKGROUND OF THE INVENTION

The current invention relates to a method for enhancing formationstability during well construction. The method of the current inventionimproves the process of formulating cementing fluids (flushes, spacers,and cement slurries) such that the fluids reduce the risk of formationinstability during well completion operations. More specifically, thecurrent invention relates to a test for determining the optimumcementing fluid formulation for use in water sensitive, reactiveformations.

Water sensitive, reactive formations include but are not limited tomarl, clay bearing sandstone, clay bearing carbonates, shale stringersin salt formations and carbonate formations. Shales are among the mostcommonly encountered formations. Shales are fine-grained sedimentaryrocks composed of clay, silt and in some cases fine sands. For thepurpose of this discussion, shale will be termed as a loosely definedheterogeneous argillaceous material ranging from clay-rich gumbo(relatively weak) to shaly siltstone (highly cemented), with the commoncharacteristic of having an extremely low permeability and contains clayminerals. Argillaceous formations like shales make up over 75 percent ofdrilled formations and cause over 90 percent of wellbore instabilityproblems. Instability in shales is a continuing problem that results insubstantial annual expenditure by the petroleum industry—in excess of abillion dollars according to conservative estimates.

A drilling fluid system (drilling mud) is an essential part of aconventional drilling process and consists of different solid and fluidcomponents. When interacting with subterranean formation material suchas shale and other water-sensitive, reactive formations, cementingfluids exhibit many of the same physical and chemical functionalitiesand properties as drilling mud. Different performance enhancingcomponents may be added to any of these fluids. As known to thoseskilled in the art, the primary functions of a drilling fluid includethe removal of rock material during drilling, imparting hydraulicsupport to the borehole to help ensure stability, providing lubricationto reduce friction between the borehole surface and drill pipe, coolingthe drill bit, etc. Cementing preflushes and spacers serve the functionof removing the drilling fluid in preparation for the cement slurry, aswell as separating potentially incompatible drilling fluids from cementslurries. Finally, the cement will serve the ultimate function of zonalisolation and structural support. In each instance, the properties ofthese fluids are adjusted to account for the changing characteristics ofwellbore formations encountered.

Cementing fluids often include several different salts (e.g. NaCl, KCl,and CaCl₂) for various purposes such as intentionally affecting(shortening) slurry set times, cementing across salt formations, andsupposed protection of productive formations that may containwater-sensitive clays. Historically, salt content in cement slurries hasvaried from one or two percent to saturation with NaCl. Use of KCl andCaCl₂ is usually limited to no more than three or four percent. Further,seawater or brine is frequently added at the wellbore location to thecement composition as makeup water to produce a cement slurry having asuitable density and pumpability.

However, the use of salts in cement slurries has not been consistentwith respect to formation issues. The position is frequently taken thatthe high pH of cement slurry, along with its minimal amount of calciumin solution, will suffice to provide formation protection in most cases.However, very little actual supporting evidence for this assumption hasbeen found. Further, most testing reported in the literature has beenbased on regained permeability testing of sandstone cores. Although verymeaningful to the understanding of that specific issue, any connectionbetween effects on clays in permeable sandstones and formationinstability as related to shales is complicated by precipitation ofvarious calcium salt species from cement slurries. The pros and cons ofthis issue are frequently debated with no clear outcome. When salts areapplied, presumably for formation stability purposes, it is frequentlydone without a true understanding of the method or outcome.Additionally, use of salts specifically in cementing spacers andpreflushes is seldom applied.

In addition to salts, there are many other additives in cementingfluids. Polymers of many types (e.g. blends containing HEC, CMHEC, andvarious synthetic polymers) as well as silicates are a frequentcomponent in cement slurries. They serve several functions includingprevention of slurry dehydration and annular bridging during placement,enhanced bonding across permeable zones, rheology adjustment, and as anaid to gas migration control. However, combining salts and fluid lossadditives in the same slurry frequently presents a more complicated andcostly scenario because many fluid loss additives do not hydrate and/orotherwise function as efficiently in the presence of high concentrationsof soluble salts. This cost-driven approach to achieving cement slurryfluid loss values has resulted in the reduction and general eliminationof salts in most primary cementing slurries without a true understandingof the resulting effects on wellbore stability.

Thus, a need exists for a method of accurately formulating cementingfluids which will enhance formation stability.

SUMMARY OF THE INVENTION

The current invention provides a method for enhancing subterraneanformation stability. In particular, the current invention is directed toa method for enhancing the stability of water-sensitive, reactivesubterranean formations. According to the method of the currentinvention, a sample of the targeted subterranean formation or a similarsubterranean formation is obtained and placed in a testing device. Thesample is placed under a confining pressure approximately equal to thepressure encountered in the subterranean formation. The confiningpressure will be maintained on the sample for the duration of the testprocedure. Additionally, back-pressure is applied on the upstream sideof the sample by a fluid similar to the fluid present in the pores ofthe subterranean formation. Subsequently, the back-pressure is releasedand the sample allowed to consolidate. Following consolidation, upstreampressure is applied to the sample while monitoring the downstreampressure exerted by the shale sample. Once the downstream pressure hasincreased by at least 50 percent, the upstream pressure is removed. Thedownstream and upstream pressures are monitored and allowed toequilibrate or at least stabilize. Thereafter, upstream pressure isagain applied to the sample by pumping a cementing fluid through thetesting device. As the cementing fluid contacts and applies pressure tothe sample, the change in downstream pressure is measured. The wateractivity of the cementing fluid is adjusted in response to the change indownstream pressure to provide an economical cementing fluid formulationhaving a positive stability enhancing impact on the subterraneanformation.

The current invention also provides a method for formulating cementingfluids to be used in a wellbore penetrating a water-sensitive, reactivesubterranean formation. The method of the current invention providescementing fluids formulated to reduce or eliminate the likelihood offormation instability during the cementing process. According to themethod of the current invention, a sample of the targeted subterraneanformation or formation of similar composition is obtained and placedunder a confining pressure in a suitable testing device. Using a fluidsimilar to formation pore fluid (simulated pore fluid), back-pressureapproximating the in situ pore pressure of the formation is applied tothe sample. Thereafter, removing the back-pressure while maintaining theconfining pressure consolidates the sample. Subsequently, upstreampressure is applied to the sample using the simulated pore fluid whilemonitoring the downstream pressure. Once the downstream pressure hasincreased by at least 50 percent, the upstream pressure is removed andthe upstream and downstream pressures are allowed to approximatelyequilibrate. Following stabilization of the upstream and downstreampressures, upstream pressure is once again applied to the sample using acementing fluid. During the application of upstream pressure, the changein downstream pressure is measured. In response to the change indownstream pressure, the water activity of the cementing fluid may beincreased or decreased. The process steps of applying upstream pressure,measuring the change in downstream pressure and adjusting the wateractivity of the cementing fluid are repeated until the downstreampressure is less than or equal to the upstream pressure. Preferably, thedownstream pressure is less than the upstream pressure.

Additionally, the current invention provides a method for enhancing thestability of a water sensitive, reactive formation penetrated by awellbore during the cementing process. The method of the currentinvention comprises formulating cementing fluids having low wateractivity levels and capable of applying an osmotic semi-permeablemembrane on the face of the formation. According to the method of thecurrent invention, a sample of the targeted subterranean formation orformation of similar composition is obtained and placed under aconfining pressure in a suitable testing device. Using a fluid similarto formation pore fluid (simulated pore fluid), back-pressureapproximating the in situ pore pressure of the formation is applied tothe sample. Thereafter, removing the back-pressure while maintaining theconfining pressure consolidates the sample. Subsequently, upstreampressure is applied to the sample using the simulated pore fluid whilemonitoring the downstream pressure. Once the downstream pressure hasincreased by at least 50 percent, the upstream pressure is removed andthe upstream and downstream pressures are allowed to approximatelyequilibrate. Following stabilization of the upstream and downstreampressures, upstream pressure is once again applied to the sample using acementing fluid. During the application of upstream pressure, the changein downstream pressure is measured. In response to the change indownstream pressure, the water activity of the cementing fluid may beincreased or decreased. Additionally, prior to or during the cementingoperation, an osmotic semi-permeable membrane is applied to the face ofthe subterranean water sensitive, reactive formation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the test cell used in themethods of the current invention.

FIG. 2 graphically represents the influence of various concentrations ofKCl on pore pressure.

FIGS. 3-6 graphically represent the influence of various additives onpore pressure during performance of the testing method of the currentinvention.

DETAILED DISCLOSURE OF THE INVENTION

The current invention relates to methods for enhancing the stability ofwater-sensitive, reactive formations such as shale formations and tomethods for accurately formulating cementing fluids. Those skilled inthe art are familiar with the benefits of incorporating various saltsand other compounds into the formulations of cementing fluids to limitthe detrimental effects of such fluids on formation stability. However,prior to the current invention, the process of formulating the cementingfluids was one of “trial and error.” The current invention provides anaccurate method for simulating the downhole environment of thesubterranean formation including the impact of cementing fluids on thesubterranean formation. By assessing the impact of cementing fluids onthe subterranean formation, the current invention provides methods forenhancing formation stability and improved methods for formulatingcementing fluids.

One critical aspect of maintaining formation stability is the balancingof fluid pressure within the pores of the formation, known as porepressure, by the fluid pressure within the borehole, known as boreholepressure. Thus, the transport of water between the borehole and theformation directly impacts the stability of the formation. In general,the two most relevant mechanisms for water transport in and out of shaleand other water sensitive formations are: (1) the hydraulic pressuredifference between the wellbore pressure (equivalent total fluid columndensity) and the formation pore pressure; and, (2) the chemicalpotential difference, i.e., water activity, between the wellbore fluidand the pore fluid within the formation. Accordingly, it is desirable tocontrol the flow of water from the wellbore into and out of theformation in order to maintain formation stability.

As will be discussed herein, the current invention utilizes two primarymethods for controlling the flow of water into and out of the formation.First, the current invention provides a method for formulating cementingfluids preferably having a water activity lower than the water activityof the pore fluid within the formation. Second, the current inventionadditionally provides for the generation of an osmotic, semi-permeablemembrane on the face of the formation.

If the cementing fluids have a higher water activity level than the porefluid within the formation, then water will flow from the wellbore intothe pores of the formation. As water enters the formation pores,formation pore pressure increases ultimately leading to formationinstability. However, if cementing fluids are formulated with a wateractivity level less than the water activity level of the pore fluids,then water will flow outwards from the formation into the wellbore. Theoutward flow of water from the formation effectively increases the fluidpressure within the wellbore thereby further stabilizing the formation.Thus, knowledge of the chemical potential of the cementing fluids, i.e.,water activity, and the impact of that potential on the formationpermits the formulation of cementing fluids that will enhance wellborestability.

As noted above, one aspect of the current invention is a method offormulating cementing fluids capable of enhancing the stability of watersensitive, reactive formations. The method of formulating the cementingfluids utilizes a testing device such as depicted in FIG. 1 to simulatethe downhole environment of the targeted formation. The testing deviceof FIG. 1 was previously disclosed in PCT Application No. WO 02/053873,published on Jul. 11, 2002 and assigned to the assignee of the currentinvention, and in the paper entitled “Development of Novel MembraneEfficient Water-Based Drilling Fluids Through Fundamental Understandingof Osmotic Membrane Generation in Shales,” by Fersheed K. Mody, Uday A.Tare, Chee P. Tan, Calum J. Drummond, and Bailin Wu, presented at theSPE Annual Technical Conference and Exhibition held in San Antonio,Tex., October 2002. Both references are incorporated herein byreference.

The method of formulating cementing fluids will be described withreference to the testing device 20 of FIG. 1. Referring to FIG. 1, theparts of the cell are shown as follows: Cell—1; Base—2; Bleed Port—3;Confining Fluid Port—4; Downstream Pressure Line (pore fluid)—5;Upstream Pressure Line (test solution)—6; Knurl—7; Top Platen—8;Membrane—9; Sample—10; Bottom Platen—11; O-Rings—12; Collar—13; andSeal—14. As shown in FIG. 1, only a single cell 1 is depicted; however,the testing device actually has six test cells. Thus, six differentformulations may be tested under simulated downhole pressure conditionsat any one time while independently controlling each individual testcell 1.

Device 20 of FIG. 1 has a confining pressure and pore pressure capacityof 35 MPa and 20 MPa respectively. The confining pressure is appliedwith a pump incorporating an accumulator (not shown) and controlled witha high precision stepping motor pump control system (not shown). Thissystem is able to control the confining pressure to within ±7 kPa of thetarget pressure. Each test cell 1 has an associated test solutioncylinder (not shown), therefore up to six different test solutions canbe tested simultaneously. This configuration also provides independentcontrol of each test start and termination. Two pore fluid cylinders(not shown) are provided to avoid interruption to the tests when thefluid runs out by switching from one cylinder to the other. Separatehigh-pressure gas cylinders (not shown) provide the upstream pressure,i.e. borehole pressure resulting from the cementing fluid, anddownstream pressure, resulting from the pore fluid. A single pressuretransducer (not shown) monitors the upstream pressure of the six testcells 1 while a separate pressure transducer (not shown) monitors thedownstream pressure of each cell 1. Circulation of the cementing fluidis adjusted with the dial gauge of a metering valve (not shown) at theupstream end of each cell. The entire device is placed in a constanttemperature facility (not shown) to control and maintain constant testtemperature.

For the purposes of the following description of the cementing fluidtesting procedure, the term “downstream pressure” corresponds to thepore pressure exerted by the targeted formation. The term “confiningpressure” refers to the in situ pressure applied to the overallformation. The term “upstream pressure” corresponds to the hydrostaticpressure exerted by fluids within the borehole. The term “back pressure”corresponds to the pore pressure of the formation as applied to the faceof sample 10 during back-pressure saturation. Back-pressure saturationis a step carried out to restore formation pore pressure and ensurecomplete saturation of sample 10 with the simulated pore fluid. Thefollowing test procedure is used to evaluate the impact of cementingfluids on the pore pressure of the formation:

-   -   1. Sample mounting—sample 10, obtained from the targeted        formation or a similar formation, is placed within test cell 1        between the platens 8 and 11. Sample 10 is jacketed in a        membrane 9, O-rings 12 are mounted over membrane 9 on platens 8        and 11 and cell 1 is filled with a simulated pore fluid.    -   2. Back-pressure Saturation—This step places sample 10 under a        pressure corresponding to the in situ formation pressure.        Typically, a confining pressure is applied to the sample while        back-pressure is applied to the upstream side of sample 10.        During this step, the downstream pressure will increase to a        pressure typically greater than the back-pressure applied. The        confining pressure is initiated at the beginning of the test and        maintained for the duration of the test.    -   3. Consolidation—The excess fluid/pressure is allowed to        drain/dissipate and sample 10 is assumed to be essentially        consolidated when the change in the downstream pressure is less        than 50 kPa/hour.    -   4. Pore Fluid Pressure Transmission—Upon consolidation of sample        10, increase the upstream pressure, applied by using the        simulated pore fluid to apply a pressure approximately equal to        the equivalent circulating density of the borehole fluid during        cementing operations at the face of the targeted formation, i.e.        this step simulates the downhole pressure experienced by the        formation. When the downstream pressure increases by more than        50 percent, reduce the upstream pressure back to sample        consolidation pore pressure.    -   5. Re-consolidation—Allow the excess pore pressure inside the        sample to dissipate such that the upstream and downstream        pressures equilibrate or at least stabilize.    -   6. Test Solution Pressure Transmission—Following the        equilibration of the downstream pressure with the upstream        pressure (or stabilization of the downstream pressure), displace        the simulated pore fluid in upstream line 6 with the test        solution, i.e. the cementing fluid. Ensure that the volume of        cementing fluid pumped is at least twice the volume of pore        fluid in the line and upstream platen. Increase the upstream        pressure to approximate the pressure exerted by the cementing        fluids on the face of the targeted formation during cementing        operations. Allow the downstream pressure to increase and        stabilize.    -   7. Displacement of Test Solution with Lower Activity        Solution—Following the equilibration of the downstream pressure        with the upstream pressure (or stabilization of the downstream        pressure), displace the cementing fluids with a cementing fluid        having a lower water activity level.    -   8. Monitor the change in downstream pressure to determine if the        water activity level of the cementing fluid will enhance        formation stability.    -   9. Continue circulation and monitor downstream pressure. Note        the maximum increase or decrease in downstream pressure. The        maximum pressure change is considered to be test termination.

As will be demonstrated by way of the following examples, use of theforegoing method enables the formulation of cementing fluids suitablefor enhancing the stability of the subterranean formation. Specifically,the foregoing method provides an accurate simulation of the downholeenvironment. Thus, the change in downstream pressure duting testing ofthe cementing fluid in device 20 reflects the relative differences inwater activity levels between the cementing fluid and the pore fluid.Accordingly, analysis of the test results will indicate whether or notthe cementing fluid has a water activity level sufficiently low enoughto enhance formation stability. If the downstream pressure is greaterthan the upstream pressure, then the water activity level of thecementing fluid must belowered. Because this method reflects thedownhole conditions of the wellbore, the water activity of the cementingfluid can be continually adjusted and re-tested until the desiredactivity is reached or no further adjustment is possible.

Adjustment of the water activity level of the cementing fluid is carriedout by adding salts and other compounds to the cementing fluid.Preferred compounds for adjusting the water activity level of thecementing fluids include but are not limited to water-soluble salts ofcalcium, sodium, potassium, magnesium and the like, and glycols and likederivatives such as ethylene glycol, propylene glycol, and othercompounds as known by those skilled in the art as being capable ofreducing water activity levels.

If the water activity level of the cementing fluid can not be loweredfurther, then additional steps to protect the formation must be taken asdescribed below. However, in many instances merely slowing the increasein pore pressure will suffice, as the cementing process will frequentlybe completed, including setting of the cement, prior to a detrimentalincrease in pore pressure.

If testing of the cementing fluid indicates that the pore pressurecannot be lowered or if the increase in pore pressure is notsufficiently retarded to protect the stability of the subterraneanformation, then additional steps must be taken to preserve the integrityof the formation. One means available for further enhancing thestability of the formation is the generation of an osmotic membrane onthe face of the formation penetrated by the wellbore. The methods ofgenerating such membranes and the effectiveness of such membranes areknown to those skilled in the art as demonstrated by the paper entitled“Development of Novel Membrane Efficient Water-Based Drilling FluidsThrough Fundamental Understanding of Osmotic Membrane Generation inShales,” by Fersheed K. Mody, Uday A. Tare, Chee P. Tan, Calum J.Drummond, and Bailin Wu, presented at the SPE Annual TechnicalConference and Exhibition held in San Antonio, Tex., October 2002 and bypublished PCT Application No. WO 02/053873, published on Jul. 11, 2002and assigned to the assignee of this invention. Both references areincorporated herein by reference.

Thus, the generation of an efficient semi-permeable membrane will helpensure adequate outflow of water from the formation or at least minimizethe flow of water into the formation due to wellbore pressure.Incorporating additives known to those skilled in the art into thecementing fluid formulation can increase the efficiency of thesemi-permeable membrane. For example, the following non-limiting list ofadditives if applied correctly may increase the efficiency of asemi-permeable membrane formed on the face of a shale formation:electrolytes, phenols, tetra methylammonium laurate, tetramethylammonium oleate, silicic acid, potassium methyl siliconate, sodiummethyl siliciconate, biopolymers, hydroxyethyl cellulose, sodiumcarboxylmethyl-hydroxethyl cellulose, synthetics such as polyethyleneamines, copolymers of 2-acrylamide-2-methyl propane sulfonic acid andN-vinyl-N-methyl acetamide, HALAD-344 (a random copolymer of2-acrylamide-2-propane sulfonic acid and N,N-dimethyl acrylamide),HALAD-413 (a caustized lignite grafted with 2-acryamide-2-methylsulfonicacid, N,N-dimethylacrylamide, and acrylamide), latexes such aspolyvinylalochol and styrene butadiene, and silicate compounds such assodium silicate and potassium silicate.

The following examples are provided merely to enhance the understandingof the current invention and are not considered limiting with regard tothe scope of the invention. Example 1 demonstrates how variations inconcentration influence pore pressure. Examples 2-6 demonstrate the useof the testing method described above as a means for evaluating theimpact of a compound on pore pressure. The following fluids were testedaccording to the foregoing method:

Fluid A—a cement filtrate produced by mixing API Class H Portland cementwith approximately 38% fresh water and extracting said filtrate fromsaid slurry in accordance with API Recommended Practice 10B, 22ndedition, December 1997.

-   -   Fluid C—a preparation of a solids-free solution to simulate the        filtrate of a cementing spacer commercially available from        Halliburton Energy Services as Tuned Spacer.    -   Fluid D.—a preparation of a sodium silicate cementing fluid        composed of 50% (by volume) fresh water and 50% (by volume)        commercially-available, 40% active sodium silicate.    -   Simulated Pore Fluid—water containing the following dissolved        salts:

Salt Concentration (g/l) NaHCO₃ 15.6 Na₂SO₄ 7.3 NaCl 3.86 Na₂CO₃ 3.30MgSO₄ 0.62 CaSO₄ 0.42

EXAMPLE 1

With reference to FIG. 2, Example 1 demonstrates how variations in saltcontent can influence pore pressure. In this example, a sample of shalewas stabilized with a solution of 8% NaCl. The sample was placed under aconfining pressure, represented by line A, of 27.6 MPa (4000 psi) andexposed to various concentrations of a KCl solution. The change in porepressure was monitored and is represented in FIG. 2 as line B. The shalewas exposed to the following concentrations of KCl solution asrepresented by lines C through G respectively: 1%, 4%, 8%, 15% and 20%.The final solution contained 8% NaCl and is represented by line H. Asreflected by line B, the pore pressure within the shale increased whenexposed to the 1% and 4% solutions of KCl due to the net imbalance insalinity. The 8% KCl solution resulted in a gradual reduction of porepressure to approximately that of the shale prior to exposure to anyupstream fluid. The 15% and 20% KCl solutions demonstrate a clearreduction in pore pressure due to a net flux of water flowing out of theshale. Thus, use of a cementing fluid having a water activity levelapproximating the water activity level of the 8% KCl solution should notdamage a downhole formation similar in structure and composition to thestructure of the shale sample tested. Finally, when exposed to the 8%NaCl solution, the pore pressure returned to the equilibrium pressure.This Example demonstrates the ability to accurately test pore pressurewithin a shale sample and the ability to adjust the salt content of afluid in order to enhance wellbore stability.

EXAMPLES 2-5

For Examples 2-5, FIGS. 3-6 depict the variation in both upstream anddownstream pressures, as represented by lines B and C respectively,during performance of the testing method described above. The confiningpressure applied to the shale sample is represented by line A. Thetemperature of the test cell is represented by line D.

The initial increase in downstream pressure reflects the performance ofthe Back-pressure Saturation step. The drop in pressure represents thesubsequent Consolidation step. Thereafter, changes in downstreampressure are influenced by the application of upstream pressure to theshale sample. On about day 2 of each test, upstream pressure (Pore FluidPressure Transmission) is applied to the sample. This increase inupstream pressure reflects the expected pressure to be applied by thecementing fluid in the downhole environment. This pressure is removedand the sample allowed to re-consolidate as reflected by the drop inpressured depicted by both lines B and C. Following reconsolidation, thesample is ready for testing by applying pressure with a cementing fluid.

The upstream pressure applied with the cementing fluid reflects theexpected borehole pressure during a cementing operation. If thedownstream pressure corresponds to the upstream pressure, as in FIGS. 3,4, 5 and 6, then a cementing fluid having a different water activitylevel is substituted for the upstream fluid. The change in downstreampressure following the change in upstream fluid reflects the impact thefluid will have on formation stability. If the downstream pressuredecreases, then the fluid should have a stabilizing effect.Alternatively, if the rate of increase in downstream pressure is atleast slowed then the fluid may preserve the formation for a period oftime sufficient to complete the cementing operations.

As used in the examples 2-5, the term KCl reflects the addition ofsufficient KCl to the cementing fluid to yield a 5% KCl solution basedon the weight of the water used to make up the cementing fluid.

Example 2, as shown in FIG. 3, demonstrates that the addition of KCl toFluid C will produce a drop in downstream pressure. Therefore, acementing fluid comprising Fluid C and KCl should have a stabilizingeffect on a water sensitive, reactive formation during cementingoperations.

Example 3, as shown in FIG. 4, reflects the use of Fluid A with andwithout KCl. The results of this test would indicate that Fluid A withKCl produces a temporary decrease in formation pore pressure with agradual increase in pressure overtime. Therefore, this test demonstratesthe ability to predict the gradual increase in formation pore pressure.As a result, the cementing operations should be completed prior to adetrimental increase in formation pore pressure.

Example 4, depicted by FIG. 5, reflects the potential impact of changingfluids during a cementing operation. This test may explain why certainformation remain stable for a portion of a cementing operation andsubsequently destabilizes at a later stage. As reflected by FIG. 5,Fluid D produces a drop in downstream pressure reflecting the drop inpore pressure within the sample. However, upon the addition of water thepore pressure spikes and continues to increase upon the subsequentaddition of Fluid A. Thus, with information of this nature, oneconducting a cementing operation will be able to predict the impact ofchanging fluids during the operation.

Example 5, as shown in FIG. 6, is a repeat of the test of Example 4;however, in this instance Fluid A also contains KCl. Although notreadily visible in the Fig. the exposure of the sample to water onceagain resulted in a jump in pore pressure. However, displacement of thewater spacer with Fluid A containing KCl stabilized the pore pressurefor several days. Subsequently, the upward trend in pore pressureresumed. However, the elapsed time should be sufficient to complete thecementing process including setting of the cement.

Thus, the current invention provides a valuable testing method fordetermining the impact of cementing fluids on water sensitive, reactiveformations. The current invention further provides the ability toformulate cementing fluids which will enhance the stability of suchformations. Additionally, the current invention provides methods forenhancing formation stability of water sensitive, reactive formationpenetrated by a wellbore during cementing operations.

While the present invention has been described in detail with referenceto FIGS. 1-6 and the examples, other embodiments of the device andmethods for performing the current invention will be apparent to thoseskilled in the art. Thus, the foregoing specification is consideredexemplary with the true scope and spirit of the invention beingindicated by the following claims.

1. A method for enhancing the stability of a water sensitive, reactivesubterranean formation penetrated by a wellbore comprising the steps of:a) obtaining a subterranean formation sample from the targeted formationor a formation of similar composition; b) placing the sample under aconfining pressure; c) applying back-pressure to the sample; d)consolidating the sample by maintaining the confining pressure whilereleasing the back-pressure from the sample; e) applying upstreampressure to the sample while monitoring the downstream pressure exertedby the sample; f) removing the upstream pressure once the downstreampressure has increased by at least 50 percent; g) allowing the upstreamand downstream pressures to approximately equilibrate; h) using acementing fluid to apply upstream pressure to the sample; i) monitoringand measuring the change in downstream pressure versus upstream pressurein response to the application of upstream pressure by the cementingfluid; and, j) adjusting water activity of the cementing fluid inresponse- to the change in downstream pressure.
 2. The method of claim1, wherein the confining pressure is approximately equal to the pressureencountered in the targeted subterranean formation.
 3. The method ofclaim 1, wherein the back-pressure applied to the sample isapproximately equal to the in situ pore pressure encountered in thesubterranean formation.
 4. The method of claim 1, wherein the step ofconsolidating the sample is considered complete when the change indownstream pressure is less than about 50 kPa/hour.
 5. The method ofclaim 1, wherein the initial application of upstream pressure is at apressure approximately equal to the hydrostatic pressure exerted on thesubject formation by fluids in the wellbore.
 6. The method of claim 5,wherein upstream pressure is reduced by about 30% when downstreampressure increases by at least 50 percent.
 7. The method of claim 1,wherein the back-pressure is applied to the sample using a simulatedpore fluid.
 8. The method of claim 1, wherein simulated pore fluid isused to apply the initial upstream pressure to the sample.
 9. The methodof claim 1, wherein the upstream pressure applied by the cementing fluidis approximately equal to the wellbore hydrostatic pressure presentduring and after cementing operations.
 10. The method of claim 1,wherein the water activity level of the cementing fluid is increased ordecreased by adjusting the concentration of compounds selected from thegroup consisting of: ethylene glycol, propylene glycol, andwater-soluble salts of calcium, sodium, potassium, magnesium, andmixtures thereof.
 11. The method of claim 1, further comprising the stepof measuring the change in downstream pressure versus upstream pressurein response to the application of upstream pressure by the cementingfluid with adjusted water activity.
 12. The method of claim 1, furthercomprising the step of continuing to adjust the water activity level ofthe cementing fluid and measuring the change in downstream pressureversus upstream pressure in response to the cementing fluid until thedownstream pressure is less than or equal to the upstream pressure. 13.A method for formulating cementing fluids to be used in a wellborepenetrating a subterranean water sensitive, reactive formationcomprising the steps of: a) obtaining a subterranean formation samplefrom the targeted formation or a formation of similar composition; b)placing the sample under a confining pressure; c) applying back-pressureto the sample; d) consolidating the sample by maintaining the confiningpressure while releasing the back-pressure from the sample; e) applyingupstream pressure to the sample while monitoring the downstream pressureexerted by the sample; f) removing the upstream pressure once thedownstream pressure has increased by at least 50 percent; g) allowingthe upstream and downstream pressures to approximately equilibrate; h)using a cementing fluid to apply upstream pressure to the sample; i)measuring the change in downstream pressure versus upstream pressure inresponse to the application of upstream pressure by the cementing fluid;j) adjusting the water activity of the cementing fluid in response tothe change in downstream pressure; and, k) repeating steps h)-j)following adjustment of the water activity of the cementing fluid untilthe downstream pressure is less than or equal to the upstream pressureexerted by the cementing fluid.
 14. The method of claim 13, wherein theconfining pressure is approximately equal to the pressure encountered inthe targeted subterranean formation.
 15. The method of claim 13, whereinthe back-pressure applied to the sample is approximately equal to the insitu pore pressure encountered in the subterranean formation.
 16. Themethod of claim 13, wherein the step of consolidating the sample isconsidered complete when the change in downstream pressure is less thanabout 50 kPa/hour.
 17. The method of claim 13, wherein the initialapplication of the upstream pressure is at a pressure approximatelyequal to the hydrostatic pressure exerted on the subject formation byfluids in the wellbore.
 18. The method of claim 17, wherein upstreampressure is reduced by about 30% when downstream pressure increases byat least 50 percent.
 19. The method of claim 13, wherein theback-pressure is applied to the sample using a simulated pore fluid. 20.The method of claim 13, wherein simulated pore fluid is used to applythe initial upstream pressure to the sample.
 21. The method of claim 13,wherein the upstream pressure applied by the cementing fluid isapproximately equal to the wellbore hydrostatic pressure present duringand after cementing operations.
 22. The method of claim 13, wherein thewater activity level of the cementing fluid is increased or decreased byadjusting the concentration of compounds selected from the groupconsisting of: ethylene glycol, propylene glycol, and water-solublesalts of calcium, sodium, potassium, magnesium, and mixtures thereof.23. A method for enhancing the stability of a subterranean watersensitive, reactive formation penetrated by a wellbore during thecementing process comprising the steps of: a) obtaining a subterraneanformation sample from the targeted formation or a formation of similarcomposition; b) placing the sample under a confining pressure; c)applying upstream pressure to the sample; d) consolidating the sample bymaintaining the confining pressure while releasing the back-pressurefrom the sample; e) applying upstream pressure to the sample whilemonitoring the downstream pressure exerted by the sample; f) removingthe upstream pressure once the downstream pressure has increased by atleast 50 percent; g) allowing the upstream and downstream pressures toapproximately equilibrate; h) using a cementing fluid to apply upstreampressure to the sample; i) measuring the change in downstream pressureversus upstream pressure in response to the application of upstreampressure by the cementing fluid; j) adjusting the water activity of thecementing fluid in response to the change in downstream pressure; and,k) prior to or during the cementing operation applying an osmoticsemi-permeable membrane to the face of the subterranean water sensitive,reactive formation.
 24. The method of claim 23, wherein the confiningpressure is approximately equal to the pressure encountered in thetargeted subterranean formation.
 25. The method of claim 23, wherein theback-pressure applied to the sample is approximately equal to the insitu pore pressure encountered in the subterranean formation.
 26. Themethod of claim 23, wherein the step of consolidating the sample isconsidered complete when the change in downstream pressure is less thanabout 50 kPa/hour.
 27. The method of claim 23, wherein the initialapplication of upstream pressure is at a pressure approximately equal tothe hydrostatic pressure exerted on the subject formation by fluids inthe wellbore.
 28. The method of claim 27, wherein upstream pressure isreduced by about 30% when downstream pressure increases by at least 50percent.
 29. The method of claim 23, wherein the back-pressure isapplied to the sample using a simulated pore fluid.
 30. The method ofclaim 23, wherein simulated pore fluid is used to apply the initialupstream pressure to the sample.
 31. The method of claim 23, wherein theupstream pressure applied by the cementing fluid is approximately equalto the wellbore hydrostatic pressure present during and after cementingoperations.
 32. The method of claim 23, wherein the water activity levelof the cementing fluid is increased or decreased by adjusting theconcentration of compounds selected from the group consisting of:ethylene glycol, propylene glycol, and water-soluble salts of calcium,sodium, potassium, magnesium, and mixtures thereof.